WhyFoldRight works here

TakeWhileByFoldRight.scala

def takeWhileR[A](s: Stream[A], f: A => Boolean): Stream[A] =
  s.foldRight(Stream[A]())((h, t) =>
    if (f(h)) Stream(h) #::: t
    else Stream.apply())

def takeWhileL[A](s: Stream[A], f: A => Boolean): Stream[A] =
  s.foldLeft(Stream[A]())((t, h) =>
    if (f(h)) Stream(h) #::: t
    else Stream.apply())

takeWhileR[Int](Stream(1,2,3,4), _ < 3).toList
takeWhileL[Int](Stream(1,2,3,4), _ < 3).toList

// I know why, and you?